For days I have been trying to use the "File" type for a column in a table on a Power Page, but the corresponding column says it is "read-only", which means no upload is possible.

This is expected behavior of the Power Page. In Power Pages, the File column is read-only initially, so you won’t be able to upload a file while creating a new record.
It will be enable once you save the record and reopen it will enable the control for file upload.
Reference Link: https://learn.microsoft.com/en-us/power-pages/configure/file-column
Workaround:
#1. You can upload file using JavaScript/WebAPI,
#2. Utilize Power Automate for upload the file.
#3. You can upload the file after record is gets created.
https://www.avatech.nz/post/another-way-to-handle-file-uploads-in-power-pages
https://francomusso.com/upload-to-a-file-column-on-record-creation-using-power-pages-portal-web-api
📩 Need more help? Just mention @Kalathiya and I’ll be happy to assist.
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